Abuffer solution with a ph of 3.80 is prepared with the same volumes of 0.72 m formic acid and m sodium formate. the ka of formic acid is
(a) 1.8x10-4.
(b) 0.82
(c) 4.0×10−8
(d) 3.4×10−5
(e) 0.41 1.6

Answer : The correct option is, (B) 0.82 M

Explanation : Given,

The dissociation constant for formic acid =

Concentration of formic acid (weak acid)= 0.72 M

pH = 3.80

First we have to calculate the value of .

The expression used for the calculation of is,

Now put the value of in this expression, we get:

Now we have to calculate the concentration of sodium formate (conjugate base or salt).

Using Henderson Hesselbach equation :

Now put all the given values in this expression, we get:

Therefore, the concentration of sodium formate is 0.82 M.